1. Field of the Invention
This invention relates to the field of amplifiers, more particularly integrated amplifiers of the MMIC type (Monolithic Microwave Integrated Circuit).
These circuits make it possible to amplify signals over a very wide frequency band (from continuous to 100 GHz) and are generally used in optical telecommunications applications.
2. Description of Related Art
FIG. 1 represents an example of a distributed amplifier. An amplifier such as this comprises a series of amplifier cells connected between two transmission lines. The one (grid line) is connected at its end to an input impedance Zin (termination), the other (drain line) is connected at its end to an output impedance Zout (termination).
Distributed amplifiers have the advantage of bypassing the frequency limitations of conventional amplifiers. For an ideal adaptation of the input and output lines, the termination impedances, Zin and Zout respectively, must have the same value as the characteristic impedance of their respective lines.
One of the problems posed by these distributed amplifiers concerns their voltage and direct current bias. As illustrated in FIG. 2, the bias voltage and the associated direct current can be supplied by a biasing circuit produced on the outside of the MMIC integrated circuit.
The biasing circuit includes a series of self-inductors connected to a voltage source in order to bring the direct voltage and current to the drain line of the distributed amplifier.
In this case, the amplifier is biased by the radiofrequency (RF) output path.
The primary difficulty is to produce such a device over a very wide frequency band (20 kHz to 100 GHz) with high current restrictions, small RF losses and good reflection factors.
In addition, the biasing circuit is cumbersome, which poses a problem when integrating it into small-size housings required for increased frequencies.
In order to eliminate these disadvantages, one solution consists of biasing the distributed amplifier through the Zout output line termination. This solution makes it possible to both satisfy the needs for a proper termination of this line and to bias the amplifier correctly.
However, for applications demanding a high output power, the distributed amplifier requires a high biasing voltage and a strong direct current. In the case of these applications, biasing the amplifier through the resistive Zout termination leads to a sharp drop in voltage at the terminals of the resistor and causes heat dissipation problems to appear.
In addition, the dimensioning of the load resistor brings with it a high stray capacitance.
This solution is therefore viable only on condition of accepting a degradation of the performance of the amplifier.
In order to overcome these difficulties, another solution consists of using an active load composed of saturable loads (field-effect transistors with their drain-source voltage saturated) for producing the Zout termination.
FIG. 3 represents a distributed amplifier including an active load such as this. The active load is composed of a set of transistors connected in parallel between a voltage source VDD and the drain line of the distributed amplifier. Each transistor has its grid connected to its source. This active load makes it possible to bias the distributed amplifier and obtain a satisfactory line termination while preventing the disadvantages linked to biasing through a resistive load.
The active load is calculated to satisfy the following conditions:VDS1+VDS2=VDDIDS1=IDS2Zca≈Zout for VDS2>VDSsatWhere VDS1 is the drain-source voltage of the amplification cell, VDS2 is the drain-source voltage of the active load, VDD is the supply voltage, IDS1 is the current delivered to the amplification cell, IDS2 is the current supplied by the active load, Zca is the impedance of the active load and VDSsat is the drain-source saturation voltage of the transistors of the active load.
One disadvantage of this solution is that the active load does not make it possible to obtain a stable Zout impedance if the direct current IDS1 (=IDS2) varies, e.g., in the case of a gain control.
When the current IDS1 becomes weaker, the transistors making up the active load can leave their saturated operating zone and operate in their linear zone. The result of this is:                that the impedance of the active load becomes weak and the condition Zca=Zout is no longer respected.        that the continuous biasing of the distributed amplifier is modified.        